1. Field of the Invention
The present invention relates to a magnetic resonance imaging method and system for imaging an object based upon a function of the spin density distribution of the specific nucleus, especially for performing surface-anatomic scanning (SAS) of the brain.
Further, the present invention relates to U.S. application Ser. No. 07/343,635 filed on Apr. 27, 1989by the same assignee of the present invention.
2. Description of the Related Art
In a magnetic resonance (MR) phenomenon, an atomic nucleus having a magnetic moment and placed in a static field absorbs or emits an electromagnetic wave having a specific frequency by resonance. A resonance frequency (Lamor frequency) .nu.O of the atomic nucleus is represented by the following equation: EQU .nu.0=.sub..gamma. H.sub.O /2 (1)
Where .gamma. is the magnetogyric ratio unique to the type of atomic nucleus, and H.sub.O is the strength of a static field.
An apparatus for diagnosing the inside of a subject by utilizing the MR phenomenon detects an electromagnetic wave generated inside the subject and processes the electromagnetic wave as an MR signal. A slice image of the subject which includes an atomic nucleus density, a longitudinal relaxation time T1, a lateral relaxation time T2, a flow, and a chemical shift can be noninvasively obtained. The MR signal can be obtained by exciting the entire subject placed in the static field. However, in practice, only a predetermined portion of the subject is excited to obtain the MR signal due to structural limitations of the apparatus and clinical requirements.
In surgical treatment for disease within the cranium of the subject, it is very important to, acquire an image representing an anatomy of a brain surface such as brain grooves or the like in order to find the position of a local cortical or subcortical diseased portion. For this reason, images are acquired by MRI. For example, when a head coil having a shape for surrounding a head of a subject is used, images including subcortical information are obtained since an MR signal for the entire head is acquired. When a surface coil is used in a prior pulse sequence (short SE), many MR signals from the subcutaneous fat and the like of the surface layer are obtained by sensibility characteristics (highly sensible to signals from a portion near the coil) of the coil. Therefore, it is difficult to accurately grasp the position of a local cortical or subcortical diseased portion in the head.
Strong demand therefore has arisen for an apparatus capable of acquiring and displaying a brain surface real anatomy to accurately find the positions of diseased portions located on the brain surface of the head of the subject.